1. Field of the Invention:
The present invention relates to a tire resonance frequency detecting apparatus which detects a tire resonance frequency from a tire vibration frequency component, and particularly relates to a tire resonance frequency detecting apparatus which can precisely detect a tire resonance frequency by specifying and reducing noise frequency components from the tire vibration frequency component.
The tire resonance frequency detected by the tire resonance frequency detecting apparatus according to the present invention can be used for a tire air pressure detecting apparatus which detects a tire air pressure on the basis of a tire resonance frequency.
2. Description of the Related Art:
In a conventional tire resonance frequency detecting apparatus, a tire resonance frequency is detected from a tire vibration frequency component which is extracted from a vehicle wheel speed signal which is generated by a wheel speed sensor or the like during vehicle traveling, and is used for a tire air pressure detection.
An example of the conventional apparatus is disclosed in Japanese Patent Application Laid-Open No. Hei. 5-133831. In this conventional tire air pressure detection apparatus, the tire vibration frequency is derived by performing a fast Fourier transform (FFT) operation with respect to the wheel speed signal. Thereafter, a noise component therein is reduced by performing an averaging process and a moving averaging process with respect to the tire vibration frequency. After that, the tire resonance frequency is calculated based on the noise-reduced tire vibration frequency.
Besides the above method, as a method of reducing the noise frequency component included in the tire vibration frequency component, a method of using a low pass filter or the like is well known.
The tire vibration frequency component, however, includes not only the resonance frequency caused by the tire air pressure but also other resonance frequencies such as a noise frequency. For example, in a tire which corresponds to a drive-wheel, a vibration thereof includes vibrations caused by a slight eccentricity of a gear in a power transmission system and unevenness of gear teeth and these become the noise frequency component.
If such a noise frequency component is included in the tire vibration frequency component, the noise frequency component may be detected as the tire resonance frequency unless it is distinguished from the tire resonance frequency. That is, there arises the possibility that a detection error of the tire resonance frequency becomes larger. Furthermore, when the tire air pressure detecting apparatus uses the tire resonance frequency including the noise frequency component to detect a tire air pressure, detection precision of the tire air pressure also deteriorates and thereby the tire air pressure detecting apparatus may warn of a fall of the tire air pressure erroneously.
To solve the above problem, the conventional apparatus merely performs the averaging process and the moving averaging process with respect to the tire vibration frequency component to reduce the noise frequency component. The above processes, however, just smooths the noise frequency component to some degree, and further a level of the true tire resonance frequency component also declines due to the averaging process and the like.
In addition, when a noise reduction process is executed by a band pass filter or the like, which the tire vibration frequency is passed through, the filter unnecessarily removes a resonance frequency which is included within the frequency band, as well as the noise frequency component.
To summarize,, because the noise frequency component included in the tire vibration frequency component cannot be specified in the noise reduction process such as the averaging process and the like, the noise reduction process which is effective to reduce the noise frequency component cannot be performed. Therefore, the detection error of the tire resonance frequency becomes larger and deteriorates the detection accuracy of the tire air pressure.